DX7 Funny Business

I’ve got an original DX7 acting up. Both the bottom and top octaves do not work. Also, buttons 1,5,8,9,13,16,17,21 and 24 do not function. What’s more, certain keys will occasionally make programs change. Any tips would be greatly appreciated. Is it possibly the AD or one of the octal buffers?

i specialize in funny business whackiness and shenanigans, your dx has told you it needs to be sent to me/its new home… it will take the process of elimination to find out which it is

I worked it out. One of the Toshiba octal buffers had went bad. Found one on eBay and she’s back to 100%. Once that was done had one key intermittently triggering, but that was a slightly oxidized key contact.

Took possession of an “as-is” TX816 yesterday, my next project. No schematics for this one afaik.


Good luck. Would enjoy hearing your results with that project.

I was lucky enough for someone to share the TF-1 schematics with me. Batteries were all either dead or dying so replaced with battery holders. Some electrolytics were bulging and replaced. Interesting Yamaha chose 16v caps on 15v rails… All working now, just a few things need to be cleaned up. A couple bad leds etc. Then it’s on to upgrading the output section.

AFAIK capacitor Voltage is not critical as long as it is equal or greater to the required value. However capacitance in units of microfarads is a critical value which must match spec exactly, as must resistors resistance in ohms, otherwise variance alters circuit characteristics and performance. Overcharged capacitors exceeding their rated Max voltage can heat, erupt, or even violently explode.

Class in session: It’s actually a good idea to derate the operating voltage of capacitors to increase their lifespan. 30-60% is a general rule of thumb.

TX816 is now completely working, and sounds really great. I might replace the PS with a switcher down the line (like I just did for a friends Matrix 1000) but that’s not pressing.

Oh, I forgot: Some of the “flickering” LED’s were due to the flux, which had not been cleaned off, becoming conductive. It was pretty grungy under the LED spacers and a few hours with isopropyl and Q-tips got them 100%. Interestingly, after fixing up a DX7, TX81Z, the aforementioned TX816 and a TX802 it seems around '88 Yamaha started cleaning the flux off of the PCB’s. Other than the TX802, the other FM synth boards were slathered with flux. Almost like conformal coating.

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